In medical practice, a variety of apparatus and techniques have been developed for treating stenotic sites within body lumens. A complication of the known treatments is a condition known as restinosis (i.e., re-narrowing) of the stenotic region following treatment. This condition can be alleviated to some degree by the use of drugs and or by implantable medical devices, namely stents.
Stents come in a variety of shapes and sizes. Generally speaking, stents provide a structure having an opening, such as a generally hollow open cylinder. Some stents provide relatively thin walls made of metal or other suitable material for in vivo implantation, the walls defining through hole, such as for the flow through of a fluid such as blood or other body fluid. Typical vascular or coronary stents are constructed of an open mesh or lattice structure and are designed to be expandable following placement within a patient's body lumen, such as an artery, to facilitate increased blood flow at the diseased location. Even with a stent in place, restinosis has been known to occur at treated sites, such as due to the occurrence of excessive tissue growth.
It is also known that if the material comprising the stent is pre-processed so that it can provide a therapeutic treatment to the arterial wall that it is in contact with, then the probability of a reoccurrence of stenosis at the location may be reduced. This desired effect has been achieved through the introduction of certain drugs or by the emission of ionizing radiation, by the stent, or by a combination of these agents.
Various techniques are known for irradiating stents, such as those described in U.S. Pat. No. 5,059,166 and U.S. Pat. No. 5,213,561. Examples of the known techniques include having a spring coil stent irradiated so that it becomes radioactive, alloying a stent spring wire with a radioactive element, such as phosphorous 32, forming a stent coil from a radioisotope core material which is formed within an outer covering, and plating a radioisotope coating (such as gold 198) onto a stent.
One disadvantage of the known manufacturing techniques is the transport time between the site of manufacture and the site of use. Because of the need for transporting stents off-site using these known techniques, at least some of the radioactive dose imparted during the manufacturing process can be lost, especially since it is desirable to use radioactive materials having relatively short half lives. In the known techniques for irradiating stent materials, it is often required to use a reactor or high power charged particle accelerator, which are not understood generally to be readily available and which may not be conveniently located to the site of medical use. In order to compensate for the undesirable transport times and distances using the known techniques, users may need to resort to materials having longer half lives, or to imparting greater radioactive doses to the stent material during manufacture, in order to compensate for the delays between manufacture and use such as in hospitals. This leads to increased inefficiency and cost.
From the above, it is apparent that there is a need for systems to handle and transport medical devices so that they are exposed to x-rays of the appropriate energy level required to generate isotopes that are emitted from known and widely available compact industrial and medical high energy x-ray sources that may be located in hospitals at sites proximate to the points of use.
Relatively lower power, and more widely available and readily accessible industrial and medical linear accelerators are also known, such as the LINATRON® and the CLINAC® linear accelerators from Varian Associates, 3100 Hansen Way, Palo Alto, Calif. 94304. These linear accelerators have been used in industry for high-energy radiography or in hospitals for clinical radiation treatments. They may provide a directed beam of high energy x-rays at structures to be analyzed or at a diseased site for therapeutic purposes. It is known that these accelerators can generate an electron beam directed at an x-ray generating target, where the energy of the electrons in the beam is converted into x-ray flux. This phenomena is known as a bremstrahlung effect and is well known in atomic and high energy physics. An example of an x-ray generating target for use with the CLINAC® medical linear accelerator is described in commonly assigned U.S. Pat. No. 5,680,433.
It is therefore an object of the present invention to provide a more economical system for irradiating target objects for use in medical applications, such as stents, using compact and efficient x-ray sources and material handling systems. It is also an object of the present invention to provide a method of making radioactive stents which can be performed at distributed sites, such as within or close to hospitals or other facilities where they may be used.
It is another object of the present invention to provide an apparatus and method for efficient irradiation of materials using available medical linear accelerators or high energy x-ray radiographic accelerators.
It is a further object of the present invention to provide increased efficiency in irradiating materials.
It is another object of the present invention to provide an apparatus and method of making radioactive stents in a manner that could be done within the hospital or facility on an as-needed basis.